A surface waveguide is a light pipe that is formed on a surface of a substrate. Surface waveguides are operative for guiding light signals along paths that can include curves, loops, etc. without a significant loss of optical energy. Typically, a surface waveguide includes a central core of substantially transparent material that is surrounded by cladding material whose refractive index that is lower than that of the core material. This refractive index difference gives rise to most of the optical energy of the signal being contained within the surface waveguide core.
Surface waveguides are typically formed on rigid substrates, such as glass or silicon. Often, multiple surface waveguides are formed on a single substrate to collectively define a planar lightwave circuit (PLC). Surface waveguides can be configured to define complex structures, such as ring resonators, 1×N couplers and splitters (where N can be 2, 3, or more), and the like, which are difficult to realize using conventional optical fibers.
The “mode” of the light signal propagates primarily within the core, although a portion (commonly referred to as the “evanescent field”) extends into the cladding. The shape of the mode and the size of the evanescent field depend strongly on the design of the surface waveguide. Factors such as surface-waveguide design (i.e., cross-sectional shape), index contrast (i.e., the effective refractive-index difference between the core and cladding), core size, and cladding thickness all impact how strongly optical energy is confined in the core, as well as the shape and size of the optical mode (i.e., mode profile and mode-field size).
In surface waveguides having only a small difference between the refractive indices (or effective refractive indices) of the core and cladding material (referred to herein as “low-contrast waveguides”), light is loosely confined in the core and the evanescent field is relatively large. The optical propagation loss of such surface waveguides can be very low; therefore, low-contrast waveguides are preferred in applications where low propagation loss is critical, such as for transmission in optical telecom or datacom systems.
Low-contrast waveguides typically exhibit optical propagation loss that is somewhat higher than that of a typical communications-grade optical fibers, but can also enable low-loss surface waveguide crossings, optical power splitting, and optical power coupling, which are difficult to achieve using optical fibers. Unfortunately, because they only loosely confine light signals, low-contrast waveguides are susceptible to severe losses at surface waveguide bends, as well as disruption from optical signals propagating in other low-contrast waveguides located nearby. Low-contrast waveguides, therefore, require large bending radii and are not well suited for use in high-density PLCs. As a result, low-contrast waveguide systems require a large chip area, which increases their cost. It is possible to design a low-contrast waveguide having a propagation mode that substantially matches the mode profile and mode-field size of an optical fiber, however. This can reduce optical loss that arises when a light signal is transferred between the surface waveguide and the optical fiber. They are attractive, therefore, for combined systems where a low-contrast waveguide is optically coupled with an optical fiber to add functionality to a low-loss optical system.
A surface waveguide having a large difference between the refractive indices (or effective refractive indices) of their core and cladding materials (referred to herein as a “high-contrast waveguide”) tightly confines a light signal in its core such that the evanescent field is relatively small. This enables high-contrast waveguides to have extremely small bending radii. High-contrast waveguides can also be located quite close to other high-contrast waveguides without incurring significant signal interference or degradation. As a result, high-contrast waveguides enable complex circuit functionality in a relatively small chip area and are well suited to large-scale integration PLCs having densely packed surface waveguides.
Unfortunately, high-contrast waveguides typically have relatively higher optical propagation loss. Their use, therefore, has historically been limited to applications in which functionality is more important than low loss, such as sensors, power splitters, and the like. In addition, the mode profile of a high-contrast waveguide is not well matched to that of an optical fiber; therefore, the optical loss that arises when a light signal is transferred between a high-contrast waveguide and an optical fiber is typically quite large. As a result, high-contrast waveguides are not well suited for combined systems where a high-contrast waveguide is optically coupled with an optical fiber.
In some cases, it is desirable to have both high-contrast waveguides and low-contrast waveguides in the same PLC. One way to enable this is through the use of a spotsize converter, sometimes referred to as a mode-field converter. In addition, a spotsize converter can enable the use of a high-contrast waveguide-based PLC with a conventional optical fiber by changing the mode profile of the high-contrast waveguide at its input and/or output to more closely match that of the optical fiber, thereby reducing fiber-to-chip coupling loss.
Attempts to form PLC-based spotsize converters in the prior art have typically relied on surface waveguide regions comprising a one-dimensional taper in the lateral dimension, wherein the lateral taper is formed using conventional photolithography and etching. Examples of such devices are described in “Optical spotsize converter using narrow laterally tapered surface waveguide for Planar Lightwave Circuits,” J. Lightwave Tech., Vol. 22, pp. 833-839 (2004). While some improvement in coupling performance is achieved with this approach, the performance and flexibility of these devices is limited because the mode-field is only controlled in one dimension.
Silicon-core surface waveguides having tapered cores have also been investigated in the prior art, such as is described in “Spotsize converters for rib-type silicon photonic wire surface waveguides,” Proceedings of the 5th International Conference on Group IV Photonics, Sorrento, Italy, September 17-19, pp. 200-202 (2008) and “Low loss shallow-ridge silicon surface waveguides,” Optics Express, Vol. 18, No. 14, pp. 14474-14479 (2010). Unfortunately, while the promise of compatibility with conventional integrated circuits is attractive, the operating wavelengths and propagation losses for silicon-core surface waveguides limit their use in many applications.
In similar fashion, optical coupling between an optical fiber and a photonic crystal surface waveguide via a laterally tapered silicon-wire surface waveguide region was demonstrated in “Spotsize converter of Photonic Crystal Surface waveguide,” NTT Technical Review, Vol. 2, pp. 36-47 (2004).
Of more promise, however, are mode-field conversion regions formed in surface waveguides that are tapered in two dimensions, such as described in “Low-Loss Compact Arrayed Surface waveguide Grating with Spot-size Converter Fabricated by a Shadow-Mask Etching Technique,” Electronics and Telecommunications Research Institute (ETRI) Journal, Vol. 27, No. 1, pp. 89-94 (2005). While the structure of these spotsize converters shows great promise for low fiber-to-chip coupling losses, shadow-mask etching is extremely difficult to control. As a result, spotsize converters fabricated in this manner are expensive to produce in volume and are likely to suffer from variations in performance as well, making them difficult, at best, to commercialize.
An improved method forming low-cost, commercially viable spotsize converters that are operable over a wide range of wavelengths would, therefore, be highly desirable.